Interpretive Summary: Escherichia coli O157:H7 is an important food-borne pathogen associated with outbreaks of bloody diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome, and foods, primarily ground beef, are major vehicles of infection. Most recently E. coli O157:H7 infections have also been linked to the consumption of mechanically tenderized steaks that were subsequently cooked to temperatures that were inadequate to kill the pathogen. Because E. coli has been associated primarily/only with ground beef products and not with intact beef the likelihood of transferring pathogens such as E. coli from the surface into the deep tissues of meat products that are mechanically tenderized has not been well studied. Beef products possibly affected would include primal/subprimal cuts that are blade tenderized or needle injected. Since these cuts will probably be perceived by both consumers and retail establishments as “like” intact steaks/roasts and as “not like” ground beef, these cuts may not be cooked to internal temperatures that would destroy any surface microbes transferred to the interior of these tenderized/restructured products. Thus, the objective of this study was to assess the extent of penetration/transfer of different levels of E. coli inoculated onto either the fat or the lean side surfaces of top butt subprimal beef cuts that were passed either once or twice through a mechanical blade tenderizer. The results demonstrated that at least 40% of the cells were transferred to the top most 1cm of the subprimal by blade tenderization, with appreciably lower levels being transferred to the deeper tissues. As such, proper cooking of steaks would be adequate for killing pathogens.

Technical Abstract:
This study assessed the penetration of Escherichia coli O157:H7 from either the lean or the fat side surface of a beef subprimal following blade tenderization. Initially, to determine the effect of varying inoculation levels on the penetration of the pathogen, subprimals were inoculated on the lean side with ca. 0.5-3.5 log10 CFU/g of a five-strain mixture of E. coli O157:H7 and then passed once through a mechanical blade-tenderizer. A second set of inoculated subprimals that were not tenderized served as positive controls. Ten core samples were removed from each lean-side inoculated, single-pass tenderized (LS) subprimal and cut into six consecutive segments starting from the inoculated side: segments 1 to 4 comprised the top four cm of the core sample and segments 5 and 6 comprised the deepest four cm. Ten cores were also obtained from control subprimals, but only the top 1 cm (segment 1) of each was sampled. Each segment was added to sterile peptone water and macerated, and the resulting slurry was surface-plated onto Sorbitol-MaConkey agar plates. Data for two trials at each inoculum level for LS subprimals were averaged. The levels of E. coli recovered from segment 1 of the control subprimals were 0.6, 1.46, 2.44, and 3.19 log10 CFU/g, respectively. The extent of penetration of the pathogen into segment 1 of tenderized subprimals at the above mentioned four initial inoculation levels was 0.22, 1.06, 2.04, and 2.7 log10 CFU/g, respectively. The levels recovered in segment 2 were at least seven-fold lower than the levels recovered from segment 1; however, depending on the inoculum level, it was possible to recover E. coli from all six segments by direct plating. In related experiments, to determine the effect of surface type and number of passes on the penetration of the pathogen, the transfer of a single inoculum level of E. coli (ca. 4 log10 CFU/g) was assessed for lean-side inoculated subprimals that were passed either once (LS) or twice (LD) through the tenderizer and for fat-side inoculated subprimals that were passed either once (FS) or twice (FD) through the tenderizer. The levels of E. coli O157:H7 recovered from segment 1 of LS, LD, FS, and FD control subprimals that were not tenderized were 3.92, 4.0, 3.79, and 4.16 log10 CFU/g, respectively. The extent of penetration of the pathogen into segment 1 for LS, LD, FS, and FD tenderized subprimals was 3.63, 3.52, 2.85, and 3.55 log10 CFU/g, respectively. The levels recovered in segment 2 were 25, 30, 14, and 50-fold lower than levels recovered in segment 1 for LS, LD, FS, and FD subprimals, respectively. However, it was possible to recover E. coli from all six segments of LS, LD, FS, and FD subprimal cores by direct plating. These results validate that mechanical tenderization can transfer E. coli O157:H7 into the deeper tissues of subprimals, but that 13- 51% of the initial surface inoculum remains in the top one cm. These data will be useful for developing blade tenderization risk assessment models and for developing safe cooking guidelines for tenderized/injected meats.